It is known to render images from volumetric imaging data, for example from volumetric medical imaging data. A set of volumetric imaging data may be referred to as an image volume. The set of volumetric imaging data may comprise a plurality of voxels with associated intensities, with each voxel being representative of a corresponding spatial location in a medical imaging scan.
Direct Volume Rendering (DVR) is a method for volume rendering. DVR may be considered to be an industry standard for volume rendering.
DVR renders a volumetric imaging data set by modelling a cloud of emissive material. Each point in a volume represented by the volumetric imaging data set is assigned properties of color and opacity. The color and opacity may be expressed as RGBA values, where R is red, G is green, B is blue and A is alpha. Alpha may be a measure of opacity in which a value of 0% indicates transparency and a value of 100% indicates full opacity.
Global illumination (GI) is a further method for volume rendering. Global illumination rendering has more degrees of freedom than DVR. In global illumination, a lighting model may be used that includes both direct illumination by light coming directly from a light source and indirect illumination, for example illumination by light that has been scattered from another surface.
In global illumination, each point in the volume is assigned a property that may be referred to as an extinction color, absorption color, or attenuation color. In the description below we use the term extinction color. The extinction color may be expressed as an RGB color.
The extinction color for a point in the volume is representative of the color-dependent absorption of light at that point in the volume. The use of extinction colors may enable realistic tissue effects due to chroma-dependent attenuation. For example, white light passing through human tissue may become redder as it passes through the tissue due to the tissue preferentially absorbing blue and green components of the white light. The use of extinction colors may allow such reddening, and other tissue effects, to be rendered.